Re: [RFC PATCH v2 02/16] mm,madvise: call soft_offline_page() without MF_COUNT_INCREASED

[Michal Hocko]

On Mon 21-10-19 07:02:55, Naoya Horiguchi wrote:
> On Fri, Oct 18, 2019 at 01:52:27PM +0200, Michal Hocko wrote:
> > On Thu 17-10-19 16:21:09, Oscar Salvador wrote:
> > > From: Naoya Horiguchi <n-horiguchi@xxxxxxxxxxxxx>
> > > 
> > > The call to get_user_pages_fast is only to get the pointer to a struct
> > > page of a given address, pinning it is memory-poisoning handler's job,
> > > so drop the refcount grabbed by get_user_pages_fast
> > > 
> > > Signed-off-by: Naoya Horiguchi <n-horiguchi@xxxxxxxxxxxxx>
> > > Signed-off-by: Oscar Salvador <osalvador@xxxxxxx>
> > > ---
> > >  mm/madvise.c | 24 ++++++++++++------------
> > >  1 file changed, 12 insertions(+), 12 deletions(-)
> > > 
> > > diff --git a/mm/madvise.c b/mm/madvise.c
> > > index 2be9f3fdb05e..89ed9a22ff4f 100644
> > > --- a/mm/madvise.c
> > > +++ b/mm/madvise.c
> > > @@ -878,16 +878,24 @@ static int madvise_inject_error(int behavior,
> > >  		 */
> > >  		order = compound_order(compound_head(page));
> > >  
> > > -		if (PageHWPoison(page)) {
> > > -			put_page(page);
> > > +		/*
> > > +		 * The get_user_pages_fast() is just to get the pfn of the
> > > +		 * given address, and the refcount has nothing to do with
> > > +		 * what we try to test, so it should be released immediately.
> > > +		 * This is racy but it's intended because the real hardware
> > > +		 * errors could happen at any moment and memory error handlers
> > > +		 * must properly handle the race.
> > > +		 */
> > > +		put_page(page);
> > > +
> > > +		if (PageHWPoison(page))
> > >  			continue;
> > > -		}
> > >  
> > >  		if (behavior == MADV_SOFT_OFFLINE) {
> > >  			pr_info("Soft offlining pfn %#lx at process virtual address %#lx\n",
> > >  					pfn, start);
> > >  
> > > -			ret = soft_offline_page(page, MF_COUNT_INCREASED);
> > > +			ret = soft_offline_page(page, 0);
> > 
> > What does prevent this struct page to go away completely?
> 
> Nothing does it.  Memory error handler tries to pin by itself and
> then determines what state the page is in now.

OK, but the page is not pinned by this context so it can go away at any
time, right? Or do I miss your point? Who would be the Error handler
context in this case?
-- 
Michal Hocko
SUSE Labs